1. Field of the Invention
The present invention relates to a light emitting diode capable of implementing high light emission efficiency by using a surface plasmon phenomenon as well as showing reduced crystal defects (i.e., threading dislocations) and a method for fabricating the same.
2. Description of the Related Art
FIG. 1 is a cross-sectional view schematically showing a layer structure of a general planar LED 10. In the figure, the LED is constituted by a substrate 1, an n-type semiconductor layer 2, an active layer 3, and a p-type semiconductor layer 4 sequentially formed from the bottom. A p-electrode 5 is formed on the top of the p-type semiconductor layer 4, while an n-electrode 6 is formed on an exposure surface of the n-type semiconductor layer 2. However, in thin-film growth for fabricating a representative GaN-based LED, the lack of materials matched with lattice constants is pointed out as one of the main problems. For this reason, a homoepitaxially grown GaN thin film has problems such as high-density defects (i.e., threading dislocations, other point defects, and the like) caused due to the mismatch in lattices and thermal expansion coefficients between a GaN film and homogeneous substrates. Up to now, since c-surface sapphire has been widely used as the substrate because it can be inexpensive and form comparatively high-quality epitaxial layer in spite of its lattice mismatch (approximately 16%).
A technology of forming a buffer layer or a shock-absorbing layer on a substrate in order to reduce the threading dislocations caused due to the mismatch is widely known, but it cannot also but generate a lot of defects. As another method, a technology such as epitaxially laterally overgrowth (ELOG) is proposed. However, overgrowth of GaN of approximately 10 μm, which is comparatively thicker is achieved in order to attach adjacent pattern regions to each other and a process cost is increased.
In relation therewith, an attempt to solve the problem of the low threading dislocations by using porous semiconductors is made. This is a technology of reducing the threading dislocations by using the porous semiconductor having a crystalline structure of an initial material as a template for homoepitaxially lateral growth of a lattice mismatch material. For example, Hartono et al. reported that a GaN layer having low defect density can be formed by using a nano-porous GaN template (Phys. Status Solidi B 244, 1793 (2007)) and that a GaN layer subsequently grown on an annealed porous GaN template shows a characteristic in that threading dislocation density was reduced by approximately 60% (Appl. Phys. Lett. 90, 171917 (2007)). Furthermore, the same researchers proposed the influence of subsequent regrowth of a GaN buffer layer on the nano-porous GaN layer at different chamber temperatures and a mechanism in which the threading dislocations in the subsequently regrown GaN film were reduced (Phys. Status Solidi C 6, No. S2, S699-S702 (2009)).
Meanwhile, in recent years, an attempt to improve internal quantum efficiency by using surface plasmon through the interaction between light and metal has been made. The surface plasmon as collective charge density oscillation which occurs on the surface of a metal thin film is a surface electromagnetic wave which is localized to a very small region which is a boundary between metal and a dielectric (air, a semiconductor, or the like), that is, an interface between both materials and progressed on the interface.
The generated surface electromagnetic wave has energy that generates resonance that varies depending on a metal type and when the generated surface electromagnetic wave is sufficiently close to an active layer which exists in an LED and the energy of the surface electromagnetic wave is matched, energy coupling occurs. In this case, energy coupling occurs in non-emission recoupling energy as well as emission recoupling energy which occurs on the active layer, such that since light is emitted by the surface plasmon, the internal quantum efficiency is increased. In general, metals such as Pd and Al are mainly used in a UV emission region and metals such as Ag, Pt, Cu, Au, and the like are mainly used in a visible ray region.
As such, recouping speed of carriers which exist in the LED is improved through the intercoupling between the surface plasmon and the active layer formed by collective oscillation of free electros which exist in metal. Therefore, an n-type GaN layer (alternately a p-type GaN layer) and an active layer (having multi-quantum well structure) are sequentially formed on the substrate and thereafter, a general metal layer is attached onto the surface of the active layer together with the p-type GaN layer (alternately, the n-type GaN layer) of a predetermined thickness (typically, approximately less than 50 nm), for the effective coupling of the active layer and the surface plasmon.
However, the related arts (Korean Patent Publication No. 2008-74474 and Korean Patent Publication No. 915502) can improve the internal quantum efficiency of the LED element through a surface plasmon resonance effect to some extent, but are still technologically limited in that the internal quantum efficiency is deteriorated due to the layer defect such as the threading dislocations.
As described above, the relates arts which are previously known merely adopt only any one of a method of using the surface plasmon and a method of reducing the defects such as the threading dislocations, and the like in order to improve the internal quantum efficiency and cannot implement both methods.